This invention relates to a switching power supply having a series-pass regulator and, more particularly, to a modification of voltages applied to the regulator for increased efficiency of operation.
Switching voltage DC power supplies represent a significant advancement in the ease of implementing an efficient switching power supply. Most electronic applications require a positive voltage step-down regulator. Such a regulator may be constructed as a MOSFET transistor (MOSFET), more specifically, a MOSFET. Such a transistor may be constructed as a p-channel or an n-channel MOSFET. A p-channel MOSFET series-pass regulator provides a more efficient operation of the power supply, but such transistor is limited in selection, both in terms of voltage and in current handling capabilities, than is the case with an n-channel MOSFET. For technical reasons, a wide variety of n-channel MOSFET""s is available in high voltage and high current capabilities. Therefore, an n-channel MOSFET would appear to be a preferred candidate for use in a series-pass regulator.
However, the n-channel MOSFET presents a problem due to an undesirably high amount of power dissipated within the MOSFET serving as the series-pass regulator. Typically, in the operation of the MOSFET as the regulator, the MOSFET is operated in either a cut-off state of zero current flow, or in a saturated state of maximum current flow between the source and the drain terminals. In the saturated state, there is the minimum threshold voltage between the gate and the source terminals which can be 5 volts of or more. The product of the saturation current times the threshold voltage represents a significant amount of power to be dissipated within the MOSFET. In contrast, in the case of a p-channel MOSFET, the power dissipated is much lower than in the n-channel MOSFET, and is due to a resistive power loss between the source and the drain terminals wherein the resistance is less than 0.1 ohm. Thus, utilization of the n-channel MOSFET introduces the problem of excessive power dissipation within the regulator element.
The aforementioned problem is overcome and other advantages are provided by a switching voltage DC power supply having a series regulator element comprising an n-channel MOSFET, wherein a drain terminal of the transistor connects with an input voltage section of the power supply and a source terminal of the transistor connects with an output voltage section of the power supply. The MOSFET operates in conjunction with additional voltage biasing circuitry which, in accordance with the invention, offsets a voltage level between the gate and the drain terminals to reduce the difference in voltage between the drain and the source terminals associated with the gate-to-source threshold voltage. As a result, the product of the current flow between the drain and the source terminals multiplied by the voltage drop between the drain and the source terminals is significantly reduced, thereby reducing the power dissipated within the series regulator element.
Three embodiments of the invention are provided. In each of the embodiments, the voltage between the drain and the gate terminals is switched between two voltage levels, wherein one voltage level terminates conduction of current through the regulator element and the other voltage level induces conduction of current through the regulator element. The voltage applied to the gate terminal includes a bias voltage established between the gate and the drain terminals to provide the desired offset in the voltage level between the gate and the drain terminals. In a first embodiment of the invention, the bias voltage is obtained from an additional winding on a transformer utilized in obtaining input DC voltage from an input AC power line. In a second embodiment of the invention, the bias voltage is obtained by modification of an output inductive filter element to include an additional winding element for providing the bias voltage. In the third embodiment of the invention, pulsations of voltage across the output inductive filter element are employed to generate the bias voltage.